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The invention relates to a method for operating a heated liquid bath
heated by a heating device, particularly an electric resistance heater,
particularly for monitoring overheating and/or a decrease in the liquid
level, wherein by means of a temperature sensor an actual temperature is
determined and, as a function of the actual temperature measured with the
temperature sensor, an alarm and/or control signal is emitted, wherein by
means of the temperature sensor the actual temperature of the heating
device or in the region of the heating device is determined. The
invention further relates to a device for carrying out said method.

Inventors:

Pforte; Dieter; (Ohlsbach, DE); Rieger; Frank; (Durbach, DE)

Assignee:

Peter Huber Kaltemaschinenbau GMBHOffenburgDE

Serial No.:

935860

Series Code:

12

Filed:

March 20, 2009

PCT Filed:

March 20, 2009

PCT NO:

PCT/EP2009/002076

371 Date:

March 15, 2011

Current U.S. Class:

340/584

Class at Publication:

340/584

International Class:

G08B 17/00 20060101 G08B017/00

Foreign Application Data

Date

Code

Application Number

Mar 31, 2008

DE

10 2008 016 442.9

Claims

1.-27. (canceled)

28. A method for operating a liquid bath (24) heated by a heating device
(12), in particular by an electric resistance heater, in particular for
monitoring for overheating and/or falling of the liquid level, wherein an
actual temperature is determined by means of a temperature sensor (16)
and an alarm signal and/or a control signal is output in dependence on
the actual temperature measured by the temperature sensor (16),
characterized in that the actual temperature of the heating device (12)
or in the region of the heating device (12) is determined by means of the
temperature sensor (16).

29. A method in accordance with claim 28, characterized in that an alarm
signal and/or control signal is output when the actual temperature of the
heating device (12) determined by the temperature sensor (16) exceeds a
preset maximum temperature, in particular a temperature lying below the
combustion point of the liquid bath by a specific amount.

30. A method in accordance with claim 28, characterized in that the
actual temperature of the liquid bath (24) is determined by a further
temperature sensor (14).

31. A method in accordance with claim 28, characterized in that an alarm
signal and/or control signal is output when the actual temperature of the
heating device (12) measured by the first temperature sensor (16) exceeds
the actual temperature of the liquid bath (24) measured by the second
temperature sensor (14) by a present first temperature value and/or by a
second temperature value.

32. A method in accordance with claim 28, characterized in that an alarm
signal and/or a control signal, in particular a fault signal, is output
when the actual temperature of the liquid bath (24) measured by the
second temperature sensor (14) exceeds the actual temperature of the
heating device (12) measured by the first temperature sensor (16) by a
preset temperature value.

33. A method in accordance with claim 28, characterized in that the
actual temperature of the heating device is determined by means of the
first temperature sensor (16) in an upper region of the heating device
(12) and additionally the actual temperature of the heating device (12)
in a lower region of the heating device (12) by means of a third
temperature sensor (52); and in that an alarm signal and/or control
signal is output in dependence on the difference of the actual
temperatures measured by the first and third temperature sensors (16,
52); and/or in that an alarm signal and/or control signal is output in
dependence on the difference of the actual temperatures measured by the
second and third temperature sensors (14, 52).

34. A method in accordance with claim 33, characterized in that a falling
of the liquid level of the liquid bath (24) is assumed when a higher
actual temperature is measured by the first temperature sensor (16) than
by the third temperature sensor (52).

35. A method in accordance with claim 30, characterized in that an empty
liquid bath (24) is assumed when the actual temperatures measured by the
first and/or third temperature sensors (16, 52) exceed the actual
temperature measured by the second temperature sensor (14) by a preset
value.

36. A method in accordance with claim 28, characterized in that, a
function test of the heating device (12) is carried out in that a brief
current pulse is applied to the heating device (12) before the actual
switching on of the heating device (12), with it being assumed that the
heating device (12) is operating properly when the first and, optionally,
the third temperature sensors (16, 52) display a temperature increase and
the second temperature sensor (14) does not display any temperature
increase.

37. A method in accordance with claim 28, characterized in that a
function test is carried out in that, with a switched off heating device
(12), the circulation pump (20) is switched on and the actual
temperatures measured by the first temperature sensor (14) and/or by the
second temperature sensor (16) and/or by the third temperature sensor
(52) are determined; and in that an alarm signal and/or a control signal
is generated when the actual temperatures measured by the first and
second temperature sensors (14, 16) and, optionally, the actual
temperature measured by the third temperature sensor (14, 52), do not
display any essentially coinciding increase and/or if there is a
difference which exceeds a preset value between at least two of the
actual temperatures after a preset time.

38. A method in accordance with claim 37, characterized in that the
heating device (12) is switched on after a preset time and a check is
made whether the temperature ramps change; and in that an alarm signal
and/or a control signal is output when no change is adopted.

39. A method in accordance with claim 28, characterized in that a
plausibility check is made in that the user is prompted to input the
combustion point of the liquid and its designation into a control device
(18) and the input combustion point is compared with the combustion point
stored in the control device (18) for the liquid under the input
designation; and in that an alarm signal is output if a deviation is
found between the input combustion point and the stored combustion point.

40. A method in accordance with claim 28, characterized in that the
control signal in particular triggers a lowering or switching off of the
heating energy for the heating device (12) after the end of a preset time
interval.

41. An apparatus (10; 50) for operating a liquid bath (24) heated by a
heating device (12), in particular by an electric resistance heater, in
particular for monitoring for overheating and/or falling of the liquid
level, having a temperature sensor (14) for determining an actual
temperature and an alarm signal unit and/or a control signal unit (18)
which is made to output an alarm signal and/or a control signal in
dependence on the actual temperature measured by the temperature sensor
(16), characterized in that the temperature sensor (16) is made for
determining the actual temperature of the heating device (12) or in the
region of the heating device (12).

42. An apparatus in accordance with claim 41, characterized in that a
second temperature sensor (14) is provided which is made for determining
the actual temperature of the liquid bath (24).

43. An apparatus in accordance with claim 41, characterized in that the
first temperature sensor (16) is arranged in an upper region of the
heating bath (12); and in that a third temperature sensor (52) is
provided which is made for determining the actual temperature of the
heating device (12) and is arranged in a lower region of the heating
device (12).

44. An apparatus in accordance with claim 42, characterized in that the
first temperature sensor (16) is arranged in an upper region of the
heating bath (12); and in that a third temperature sensor (52) is
provided which is made for determining the actual temperature of the
heating device (12) and is arranged in a lower region of the heating
device (12).

45. An apparatus in accordance with claim 41, characterized in that the
first temperature sensor (16) is arranged above the designated liquid
level (44).

46. An apparatus in accordance with claim 42, characterized in that the
first temperature sensor (16) is arranged above the designated liquid
level (44).

47. An apparatus in accordance with claim 42, characterized in that the
second temperature sensor (14) is arranged spatially remote from the
heating device (12), in particular in the lower region of the liquid bath
(24), preferably at a circulation pump (20).

48. An apparatus in accordance with claim 41, characterized in that the
first temperature sensor (16) and, optionally, the third temperature
sensor (52) are arranged directly at the heating device (12), are in
particular thermally coupled thereto.

49. An apparatus in accordance with claim 42, characterized in that the
first temperature sensor (16) and, optionally, the third temperature
sensor (52) are arranged directly at the heating device (12), are in
particular thermally coupled thereto.

50. An apparatus in accordance with claim 48, characterized in that the
first and, optionally, the third temperature sensor (16, 15) are made for
measuring the film temperature of the bath liquid at the surface of the
heating device (12).

51. An apparatus in accordance with claim 49, characterized in that the
first and, optionally, the third temperature sensor (16, 15) are made for
measuring the film temperature of the bath liquid at the surface of the
heating device (12).

52. An apparatus in accordance with claim 43, characterized in that the
first and the third temperature sensors (14, 52) are arranged at the
hottest points of the heating device (12).

53. An apparatus in accordance with claim 44, characterized in that the
first and the third temperature sensors (14, 52) are arranged at the
hottest points of the heating device (12).

54. An apparatus in accordance with claim 43, characterized in that the
alarm and/or control signal unit (18) is made to output an alarm signal
and/or a control signal in dependence on the difference of the actual
temperatures measured by the first and second temperature sensors (14,
16) and/or by the first and third temperature sensors (16, 52).

Description

[0001] The present invention relates to a method for operating a liquid
bath heated by a heating device, in particular by an electric resistance
heater, in particular for monitoring for overheating and/or falling of
the liquid level, wherein an actual temperature is determined by means of
a temperature sensor and an alarm signal and/or a control signal is
output in dependence on the actual temperature measured by the
temperature sensor. The present invention furthermore relates to an
apparatus for operating a liquid bath heated by a heating device, in
particular by an electric resistance heater, in particular for monitoring
for overheating and/or falling of the liquid level, having a temperature
sensor for determining an actual temperature and an alarm signal unit
and/or a control signal unit which is made to output an alarm signal
and/or a control signal in dependence on the actual temperature measured
by the temperature sensor.

[0002] Apparatus of the named kind are known and are also called
temperature regulators, thermostats or bath thermostats. The liquid in
the liquid bath, which is also called a heat carrier, can be combustible.
In accordance with the standards DIN EN 61010-2-010 and DIN 12879, such
an apparatus must then be equipped with an adjustable over-temperature
protection and a low-level protection.

[0003] The over-temperature protection is also called an over-temperature
limiter. It regulates the heating energy supplied to the heating device
such that the actual temperature measured by the temperature sensor does
not reach and in particular does not exceed the combustion point of the
liquid in the liquid bath. Since the liquid bath is normally circulated,
the measured temperature as a rule corresponds to a mean temperature of
the liquid bath. It can therefore not be ensured that temperatures occur
in specific regions or in the environment of the liquid bath which are
higher than the actual temperature measured with the temperature sensor.

[0004] In known apparatus, floats are used as low-level protection in the
liquid bath with whose aid the position of the liquid level is
determined. A fall in the liquid level beneath the designated liquid
level can be determined from this. It is, however, problematic, that a
float can because caught and so can no longer serve for the determination
of the then current liquid level of the liquid bath.

[0005] It is the object of the present invention to provide an improved
apparatus and an improved method of the initially named kind.

[0006] This object is satisfied in a method of the initially named kind in
that the actual temperature of the heating device or in the region of the
heating device is determined by means of the temperature sensor. In an
apparatus of the initially named kind, the object is satisfied in that
the temperature sensor is made for determining the actual temperature of
the heating device or in the region of the heating device.

[0007] The highest temperatures in the liquid bath occur at the heating
device or in the environment of the liquid bath. In the method in
accordance with the invention, a monitoring of the actual temperature of
the heating device takes place by means of the temperature sensor. It can
thus be ensured that even the hottest points in the region of the liquid
bath do not reach the combustion point of the liquid.

[0008] An alai in signal and/or control signal is/are preferably output
when the actual temperature of the heating device determined by the
temperature sensor reaches a preset maximum temperature. The maximum
temperature in this respect can be selected so that, for example, it lies
15 K below the combustion point of the liquid. In particular a reduction
in the heating energy which is supplied to the heating device can be
triggered by means of the control signal. It can thereby be ensured that
the actual temperature of the heating device falls and also that no
temperature is reached at the heating device which causes an inflammation
of the liquid.

[0009] A conventional second temperature sensor for determining the actual
temperature of the liquid bath is preferably provided in addition to the
temperature sensor at the heating device. The bath temperature can
thereby be determined directly and does not have to be derived from the
temperature of the heating device.

[0010] In accordance with a further preferred embodiment of the invention,
an alarm signal and/or a control signal is output when the actual
temperature of the heating device measured by the first temperature
sensor exceeds the actual temperature of the liquid bath measured by the
second temperature sensor by a preset first temperature value, for
example 15K, and/or by a second temperature value, for example 30 K. The
difference in the actual temperatures between the heating device and the
liquid bath can hereby be monitored and, if it exceeds the first
temperature value, a response, for example a limiting of the heating
energy, can be provided by output of the alarm signal and/or control
signal. If the second temperature is exceeded, a fault can be output, for
example.

[0011] Preferably, furthermore, an alai in signal and/or a control signal,
in particular a fault signal, is output when the actual temperature of
the liquid bath measured by the second temperature sensor exceeds the
actual temperature of the heating device measured by the first
temperature sensor by a preset temperature value. If the measured actual
temperature of the liquid bath is higher than the measured actual
temperature of the heating device, a malfunction of the first temperature
sensor and/or of the second temperature sensor is present, which can be
indicated via the fault signal.

[0012] The first temperature sensor is preferably arranged in an upper
region of the heating device and directly at the heating device. There is
in particular a good thermal coupling between the first temperature
sensor and a critical point of the heating device. This point can, for
example, be the upper end of a heating bar of or a heating coil forming
the heating device.

[0013] Provision can in particular be made that the upper region of the
heating device projects above the liquid when the liquid bath is filled
up to the designated level. Since the heating device as a rule outputs
heat less effectively to the environmental air than to the liquid, the
upper region will heat up more than the region of the heating device
present in the liquid. Consequently, the highest temperatures, which are
monitored by means of the first temperature sensor, will occur in the
upper region of the heating device.

[0014] In another respect, an inflammation of the liquid is in particular
only possible when the liquid has contact with oxygen and a liquid/oxygen
mixture can form. It is therefore precluded by the method in accordance
with the invention by the arrangement of the first temperature sensor in
the upper region of the heating device that temperatures are particularly
reached there, in particular after the falling of the liquid level, which
result in an ignition of the liquid/oxygen mixture.

[0015] A third temperature sensor for determining the actual temperature
is preferably further preferred which is arranged in a lower region of
the heating device and directly at the heating device. An alarm signal
and/or control signal is output in dependence on the difference of the
actual temperatures measured by the first and third temperature sensors.
An alarm signal and/or control signal can thus in particular be output on
too high a temperature drop between the upper region and the lower region
of the heating device. Provision can in particular be made to output an
alarm signal and/or a control signal when the difference of the actual
temperatures is 15 K or 30 K.

[0016] An alarm signal and/or control signal can furthermore be output in
dependence on the difference of the actual temperatures measured by the
second and third temperature sensors. A redundancy is hereby achieved by
which it is also ensured on a failure of the first temperature sensor
that the difference between an actual temperature at the heating and the
actual temperature of the liquid bath is determined.

[0017] In accordance with a further embodiment of the invention, a fall in
the liquid level of the liquid bath is assumed when a higher actual
temperature is measured by the first temperature sensor than by the third
temperature sensor. As already mentioned further above, the upper region
is first more highly heated than the lower region of the heating device
on the falling of the level so that the falling can be determined
particularly advantageously by comparison of the actual temperatures of
the heating device measured by the first and third temperature sensors.

[0018] Optionally, the heating energy which is supplied to the heating
device is preferably lowered or switched off when the difference of the
actual temperatures measured by the first and third temperature sensors
is greater than or equal to a preset third temperature value, in
particular 15 K. Further preferably, an alai in signal is output when the
difference is greater than or equal to a preset fourth temperature value,
in particular 30 K.

[0019] Furthermore, an essentially empty liquid bath can be assumed when
the actual temperatures of the heated heating device measured by the
first and/or third temperature sensors exceed the actual temperature
measured by the second temperature sensor. On an empty liquid bath, the
second temperature sensor will measure the temperature of the
environmental air which remains essentially constant. In contrast, the
first temperature sensor and, optionally, the third temperature sensor
measure the actual temperatures of the heated heating device. In this
respect, a fallen or empty liquid bath can be detected by means of the
method in accordance with the invention by comparison of the actual
temperatures measured by the first, second and/or third temperature
sensors. A float in the liquid bath is no longer needed.

[0020] Particularly preferably, a check for an empty liquid level, as
described above, is carried out on every switching on of the heating
device. A putting into operation with a still empty bath can thereby be
prevented.

[0021] Further preferably, on the switching on of the heating device, a
check is made whether the heating device is working properly in that a
brief current pulse is applied to the heating device. If the first and
third temperature sensors report a temperature increase, but not the
second temperature sensor, it can be assumed that the heating device is
working properly.

[0022] In accordance with a further preferred embodiment of the invention,
a circulation pump is provided for the circulation of the liquid in the
liquid bath. A function test of the heating device is preferably carried
out with this in that the circulation pump is switched on with a switched
off heating device and a determination is made whether the actual
temperatures measured by the first temperature sensor and/or the second
temperature sensor and/or the third temperature sensor over a preset time
period indicate a ramp-like increase after the switching on of the
circulation pump which is caused by the heat development of the
circulation pump.

[0023] The operability of the first temperature sensor and of the second
temperature sensor and, optionally, of the third temperature sensor is in
particular assumed if the measured actual temperature ramps with a still
unheated heating device substantially correspond and/or if, after a
preset time, there is no difference between the measured actual
temperatures.

[0024] If a discrepancy is detected between the measured ramps or actual
temperatures, the operational check of the first temperature sensor and
of the second temperature sensor and/or of the third temperature sensor
can again be carried out after a preset time period. If then a
discrepancy is again detected between the ramps or actual temperatures,
an alarm signal is preferably output, in particular to inform the user of
a possible inoperability of the temperature sensors.

[0025] In accordance with a further preferred embodiment of the invention,
the combustion point of the liquid is interrogated from a user in that
the user is prompted to input the combustion point of the liquid into a
control device. Alternatively or additionally, the user can be prompted
to input the exact designation of the liquid into the control device. The
combustion points for a plurality of liquids are preferably stored in the
control device so that the control device can determine the combustion
point for the designation input by the user. Further preferably, a
plausibility check is made whether the input combustion point corresponds
to the combustion point stored for the liquid in the control device in
that the combustion points are compared with one another. Optionally, an
alarm signal is output if a deviation is detected between the input
combustion point and the stored combustion point.

[0026] The basic idea of the invention therefore comprises attaching a
temperature sensor in the direct proximity of the heating device.
Particularly preferably, two sensors are provided, and indeed one in the
upper region of the heating device and one in the region of the lower end
of the heating device. The bath sensor for maintaining the bath
temperature is preferably attache as usual in the proximity of the
circulation pump. The temperature sensors attached to the heating body
are preferably attached so close to the heating body that the film
temperature of the liquid film formed at the heating body can be
measured. The temperature sensors can in particular also be in direct
contact with the heating body.

[0027] It cannot only be determined with reference to the two or three
measured values of the temperature sensors whether an overheating of the
heating bath occurs, but a level fall of the bath can also be determined.
If, for example, the determined temperature at the upper sensor and at
the lower sensor at the heating device increases pronouncedly, while the
bath sensor does not detect any temperature increase, then the bath is
empty since the bath sensor is measuring the environmental temperature.
If only the temperature at the upper heating sensor increases, but not at
the lower heating sensor, an overheating is present which is due to a
fall in the level of the liquid.

[0028] An alarm is output on detecting a fault, in particular an
overheating or a level drop of the bath. Provision can also be made to
switch off the heating device, with this preferably taking place after a
settable time so that an operator can optionally top up liquid in good
time and an interruption of the process is not necessary. It is also
possible to lower the heating energy on detecting an impending
overheating. An interruption of the process can also thereby be avoided.
In another respect, the heating coil is conducted upwardly beyond the
designated liquid level in a special embodiment and is heated, with the
measurement taking place at the upper end in this region. An overheating
can thereby be detected at an early time.

[0029] In accordance with a further idea, a safety check can be carried
out on the switching on of the bath whether the heating is working
properly without the temperature adjustment having to be started. A brief
current pulse is applied to the heater for this purpose. If the upper
temperature sensor and the lower temperature sensor at the heating device
report a temperature increase, but not the bath sensor, then the heater
is in order. A signal is then output that the bath is empty.

[0030] All the described measures can be integral to the invention both
individually and in any desired combination with one another.

[0031] The invention will be described in the following by way of example
with reference to advantageous embodiments and to the enclosed drawings.
There are shown:

[0032] FIG. 1 schematically, a first variant of an apparatus in accordance
with the invention;

[0033] FIG. 2 schematically, a second variant of an apparatus in
accordance with the invention.

[0034] The apparatus 10 shown in FIG. 1 has a heating device 12, a first
temperature sensor 16, a second temperature sensor 14, a control device
18 and a circulation pump 20. The heating device 12 and the circulation
pump 20 are arranged in a container 22 for a liquid bath 24. Furthermore,
the second temperature sensor 14 is arranged at the circulation pump 20
and the first temperature sensor 16 is arranged in an upper region
directly at the heating device 12.

[0035] The control device 18 includes a regulation device 26, a
microprocessor 28, a memory 30, an input device 32 which is a keypad, for
example, and an output device 34 which is a screen, for example.

[0036] The heating device 12, the first temperature sensor 16, the second
temperature sensor 14 and the circulation pump 20 are connected to the
control device 18 in each case by means of electric leads 36, 38, 40, 42
so that they can be controlled and/or supplied with current by the
control device 18.

[0037] Provision is in particular made to fill the liquid bath 24 with
liquid up to a designated liquid level 44. The heating device 12 and the
circulation pump 20 are then completely immersed in the liquid bath 24.
The upper end of the heating device 12 can, however, also project
somewhat above the designated liquid level and the first temperature
sensor can be arranged in this region of the heating device 12 disposed
outside the liquid bath.

[0038] A user of the apparatus 10 in accordance with the invention can
input the temperature to which the liquid should be heated in the liquid
bath 20 into the control device 18 via the input device 32. Furthermore,
the user can input the combustion point of the liquid in the liquid bath
24 via the input device 32.

[0039] Subsequently, the circulation pump 20 is first switched on by the
control unit 18 so that the liquid bath 24 is circulated. Then the
regulation device 26 regulates the heating device 12 by supplying
electrical heat energy and using the actual temperature of the liquid
bath 24 measured by means of the second temperature sensor 14 so that
said liquid bath is heated to the desired temperature set by the user. In
this process, the regulation device 26 monitors the actual temperature of
the heating device 12 by means of the first temperature sensor 16.

[0040] The actual temperature measured at the heating device 12 by means
of the first temperature sensor 16 can he higher than the actual
temperature which is measured by the second temperature sensor 14 and
which substantially corresponds to a mean temperature of the liquid bath
24 due to the circulation of the liquid bath 24. In this respect, it can
be possible that the actual temperature of the heating device 12
increases for heating the liquid bath 24 so that said actual temperature
comes into the proximity of the combustion point of the liquid bath 24.
This can in particular be the case if the desired temperature lies in the
proximity of the combustion point.

[0041] The regulation device 26 is made so that it outputs a control
signal which triggers a reduction or, optionally, also a switching off,
of the supply of the heat energy to the heating device 12 when the actual
temperature of the heating device 12 measured by means of the first
temperature sensor 16 exceeds a preset maximum temperature.

[0042] The maximum temperature is in this respect fixed by the processor
28 with reference to the combustion point of the liquid bath 24 specified
by the user, for example so that it is 15 K below the combustion point.
It is thus ensured by the control signal that the actual temperature of
the heating device 12 is lowered on reaching the maximum temperature and
the combustion point of the liquid bath 24 is not reached. An
inflammation of the liquid bath 24 is thus avoided.

[0043] The regulation device 26 is furthermore made so that it triggers a
lowering or switching off of the heating energy which is supplied to the
heating device 12 when the difference between the actual temperature
measured by the first temperature sensor 16 and the actual temperature
measured by the second temperature sensor 14 exceeds a specific value,
for example 15 K or 30 K. It is thus ensured that too high a temperature
difference does not occur between the heating device 12 and the liquid
bath 24 heated thereby. An overshoot of the actual temperature beyond the
desired temperature set by the user is in particular avoided by this
measure.

[0044] Furthermore, the control device 18 is made to output a fault signal
via the output device 34 when the difference between the actual
temperature measured by the second temperature sensor 14 and the actual
temperature measured by the first temperature sensor 16 exceeds a
specific positive value. Basically, with a heated heating device 12, the
actual temperature measured by the first temperature sensor 16 should be
larger than the actual temperature of the liquid bath 24 which is
measured by the second temperature sensor 14. In this respect, a fault or
a malfunction of the first and/or second temperature sensors 14, 16 can
be assumed when this difference exceeds the specific value, for example 3
k, while taking account of a tolerance range. In this case, the control
device 18 can in particular output an alarm signal via the output device
34 to draw the attention of the user to a possible malfunction of the
temperature sensors 14, 16.

[0045] The apparatus 50 shown in FIG. 2 has the elements known from the
apparatus 10 for which the same reference numerals are used in FIG. 2.
The apparatus 50 furthermore has a third temperature sensor 52 which is
connected to the control device 18 via the electric leads 54 so that said
temperature sensor can be controlled and/or have a current supplied by
the control device 18. The third temperature sensor 52 is arranged in the
lower region directly at the heating device 12.

[0046] The apparatus 50 carries out the measured described further above
with respect to the apparatus 10 to monitor the liquid bath 24 for
overheating by means of the first and second temperature sensors 14, 16.
The regulation device 26 in the apparatus 50 furthermore compares the
actual temperature of the heating device 12 which is measured by the
third temperature sensor 52 with the actual temperature of the liquid
bath 24 which is measured by the second temperature sensor 14 and outputs
a control signal which triggers a reduction or switching off of the
heating energy which is supplied to the heating device 12 when the
difference in the measured actual temperatures exceeds a specific value,
for example 15 K. A redundancy can thereby be achieved in addition to the
monitoring of the heating device by the first temperature sensor 16. The
regulating device 26 can also compare the actual temperatures measured by
means of the first temperature sensor 16 and of the third temperature
sensor 52 with one another and output a control signal in dependence on
the difference of these actual temperatures. In this respect, in
particular a reduction in the heating energy supplied to the heating
device 12 can be effected if this difference exceeds a specific value,
for example 15 K or 30 K to counter the effect of overheating in the
upper region of the heating device 12.

[0047] The regulation device 26 can in this respect in particular be
configured so that it is avoided with the greatest priority that an
actual temperature measured at the heating device 12 by means of the
first or third temperature sensors 16, 52 reaches the combustion point of
the liquid and that in particular an alarm signal and/or a control signal
is triggered which triggers a reduction in the heating energy supplied to
the heating device 12 when an actual temperature measured at the heating
device 12 reaches or exceeds the preset maximum temperature.

[0048] The regulation device 26 can trigger an alarm signal or control
signal with the second greatest priority which in particular lowers the
heating energy supplied to the heating device 12 when the difference
between the actual temperature of the heating device 12 which is measured
by the first temperature sensor 16 and the actual temperature of the
liquid bath 24 which is measured by the second temperature sensor 14
exceeds a preset value, for example 15 K. The second highest priority can
in this respect also only be optionally taken into account by the
regulation device.

[0049] The regulation device in addition regulates the heating device 12
with the third highest priority such that a heating of the liquid bath 24
to the desired temperature set by the user takes place.

[0050] The liquid level of the liquid bath 24 can differ from the present
liquid level 44. For example, the liquid bath can have a fallen liquid
level 56. With the fallen liquid level 56, the upper region of the
heating device 12 in which the first temperature sensor 16 is arranged is
no longer flowed around by liquid. This region is rather surrounded by
the environmental air. The heat generated by the heating device 12 is
dissipated less effectively to the environmental air than to the liquid.
The upper region of the heating device 12 is thereby heated more than the
lower region of the heating device 12 which is flowed around by liquid.
With a fallen liquid level 56, the first temperature sensor 16 will
therefore display a higher actual temperature than the third temperature
sensor 52.

[0051] The control device 18 makes use of this in that it assumes a drop
in the liquid level when a higher actual temperature is measured by the
first temperature sensor 16 than by the third temperature sensor 52. The
regulation device 26 in particular triggers a lowering or switching off
of the electrical heating energy supplied to the heating device 12 when
the difference of the actual temperatures measured by the first
temperature sensor and the third temperature sensor is larger than or
equal to a preset temperature value which amounts, for example, to 15 K
or more. The regulation device 26 can also output an alarm signal via the
output device 34 in order thus to indicate to the user that the liquid
level has presumably fallen.

[0052] It can furthermore occur that the apparatus 50 is operation with an
essentially empty liquid bath 24. All three temperature sensors 14, 16,
52 are then flowed around by the environmental air. With a heated heating
device 12, the actual temperatures measured by means of the first
temperature sensor 16 and of the third temperature sensor 52 will
therefore increase, whereas the essentially constant temperature of the
environmental air is measured by the second temperature sensor 14. These
criteria are used by the regulation device 26 to recognize an empty
liquid bath. The regulation device 26 in particular triggers a control
signal to switch off the heating device after the end of a preset time
after the regulation device assumes a drying out of the liquid bath 24.
In addition, the regulation device 26 can be made to output an alarm
signal for a user to indicate to him that the liquid bath 24 is empty and
optionally to prompt him to top up liquid.

[0053] The regulation device 26 is preferably made to carry out the
above-described test for drying out or for falling of the liquid level
before the start of a temperature control of the heating device 12. The
attention of a user can thereby already be drawn to a fallen or empty
liquid bath 24 at the start of the temperature control.

[0054] The regulation device 26 can furthermore be made so that a test is
made before the heating device 12 is put into operation whether the
temperature sensors 14, 16, 52 work properly. For this purpose, the
circulation pump 20 is switched on and the actual temperatures are
determined by means of the three temperature sensors 14, 16, 52 after the
end of a preset time after the switching on of the pump 20, for example
after 10 seconds. The three temperature sensors 14, 16, 52 should then
essentially indicate the same temperature and/or the same temperature
ramp over the preset time since the liquid bath 24 is heated by the
circulation pump 20. If a deviation of the actual temperatures displayed
by the three temperature sensors 14, 16, 532 beyond a fixed confidence
interval, for example 3 K, is found, the regulation device 26 can be
configured so that it measures the actual temperatures or ramps again
using the three temperature sensors after the end of a further time
interval. if the three actual temperatures or ramps then still do not
coincide, it is assumed that at least one of the temperature sensors 14,
16, 52 is inoperable and the control device 18 outputs a corresponding
fault report to the user by means of the output device 34.

[0055] As already mentioned, the user can, on the one hand, set the
desired temperature to which the liquid bath 24 should be heated. On the
other hand, the user is prompted by the control device 18 to input the
combustion point for the liquid in the liquid bath 24. The user can
further be prompted by the control device 18 to input an exact
designation for the liquid which is in the liquid bath 24. An association
between exact designations of a plurality of liquids and their combustion
points can be stored in the memory 30. The processor 28 can then, on the
basis of the designation set by the user, read out the combustion point
stored for it in the memory 30 and can compare it with the combustion
point input by the user. A plausibility check can hereby take place
whether the user has actually input the correct combustion point. The
control device 18 can in particular output an alarm signal to the user if
a discrepancy is found between the input combustion point and the stored
combustion point.